Place corned beef brisket, fat side down, in a roasting pan with about 1/2 inch of water.

Seal the pan tightly with heavy foil.

Cook in a preheated oven at 350°F (175°C) for 2 1/2 to 3 hours, or until the brisket is tender, ensuring a small amount of water remains in the pan.

Once the brisket is cooked, remove it from the oven and carefully drain the liquid from the roasting pan into a large stock pot.

In a separate bowl, create a paste by combining brown sugar and prepared mustard.

Generously spoon the brown sugar and mustard glaze over and under the cooked brisket.

Return the glazed brisket to the oven.

Continue cooking at 350°F (175°C) for an additional 30 minutes, allowing the glaze to caramelize and adhere to the meat.